1. Field of the Invention
The present invention relates to a method of driving an ink-jet head for discharging ink droplets to record an image on a recording medium, and an ink-jet recording apparatus.
2. Description of the Related Art
Conventionally, there has been known an ink-jet type recording apparatus for recording a character and an image on a recording medium by employment of an ink-jet head having a plurality of nozzles for discharging ink. FIG. 1 is a schematic front view showing an ink-jet head 100, FIG. 2 is a schematic cross-sectional view thereof, and FIG. 3 is an exploded diagram showing a periphery of a driving part for generating a pressure necessary for discharge of ink and of a nozzle part from which ink is finally discharged.
As shown in FIG. 3, in a piezoelectric ceramic plate 1, a plurality of grooves 5 are arranged in parallel with each other, and the grooves 5 are separated from each other by side walls 7. One end of each of the grooves 5 in a longitudinal direction thereof extends to one end surface of the piezoelectric ceramic plate 1. The other end thereof does not extend to the other end surface of the piezoelectric ceramic plate 1, and a depth of each of the grooves 5 gradually decreases. In addition, on the side walls 7 on both sides of each of the grooves 5 on the opening side, there are formed an electrode 4 and an electrode 9 for driving electric field application in the longitudinal direction.
Further, on the opening side of the grooves 5 of the piezoelectric ceramic plate 1, there is formed a head chip 26 which is joined to an ink chamber plate 2. To the end surface at which the grooves 5 of a joined body of the piezoelectric ceramic plate 1 and the ink chamber plate 2 are opened, a nozzle plate 3 is joined. In the nozzle plate 3, a plurality of nozzle holes 11 are formed at positions opposite to every other groove 5. The nozzle plate 3 and the head chip 26 are each fixed by a head cap 12. The electrode 4, the electrode 9, and a drive circuit substrate 14, which are formed on the head chip 26, are electrically connected to each other via a flexible substrate 19.
Further, on the ink chamber plate 2, an ink flow path 21 for supplying ink to each of the grooves 5 is fixed, an ink inlet 41 for introducing ink is formed at a central portion of the ink flow path 21, and the ink inlet 41 is connected to a pressure absorbing unit 20 for absorbing a pressure fluctuation caused during a printing operation.
Next, a method of driving the ink-jet head 100 structured as described above will be described with reference to FIGS. 20A to 20C, 4A, and 4B. FIGS. 20A to 20C are diagrams each showing a discharge signal waveform of the ink-jet head 100 according to a related art. FIGS. 4A and 4B are cross-sectional diagrams each showing a wiring state of the electrodes of the ink-jet head 100. FIG. 20A is a diagram showing a discharge signal waveform in a case of discharging ink with a volume of one droplet according to the related art. FIG. 20B is a diagram showing a discharge signal waveform in a case of discharging ink with a volume of two droplets according to the related art. FIG. 20C is a diagram showing a discharge signal waveform in a case of discharging ink with a volume of three droplets according to the related art. FIG. 4A is a cross-sectional diagram of the ink-jet head when the ink-jet head is not driven, and FIG. 4B is a cross-sectional diagram of the ink-jet head when the ink-jet head is driven. The arrow 6 indicates a polarization direction. When an electric field is applied to the electrode 4 and the electrode 9 which sandwich the side wall 7, each of the side walls 7 deforms in a desired direction. In other words, the side walls 7 are each structured as an actuator which is deformed and operated in response to an applied voltage to be applied to each of the electrode 4 and the electrode 9.
As shown in FIG. 4A, the ink-jet head 100 has an electrode structure in which the electrode 4 formed in each of the grooves 5 is a common electrode with a ground potential, and the electrodes 9 sandwiching the electrode 4 are each applied with a drive pulse from an outside. When a positive electric field pulse, which is represented by a discharge signal waveform for the ink with the volume of one droplet as shown in FIG. 20A, is applied to each of the electrodes 9, the side walls 7 are each deformed due to a potential difference between the electrode 9 and the electrode 4 as shown in FIG. 4B. The side walls 7 are each deformed for a time T1b during which the positive electric field is applied to each of the electrodes 9. When a potential of the electrode 9 becomes 0 after the elapse of the time T1b, the side walls 7 each return to a state shown in FIG. 4A again. Note that the time T1b is set as a most efficient time at which the discharge speed is increased as being apparent from FIG. 19 showing a relation between an electric field application time and a discharge speed. Due to the deformation of each of the side walls 7, the ink filled in each of the grooves 5 changes in pressure, whereby one ink droplet is allowed to fly from the nozzle hole 11.
Further, the positive electric field is applied a plurality of times so as to change a discharge volume of the ink flying onto the recording medium from each of the nozzle holes 11, thereby making it possible to perform gradation expression. For example, in order to discharge the ink with the volume of two droplets from each of the nozzle holes 11, the positive pulse (application time T2b) is operated before the positive electric field pulse (application time T1b) during an interval of a time T4b as shown in FIG. 20B. In a similar manner, in the case of discharging the ink with the volume of three droplets, the positive electric field pulse (application time T3b) is operated before the positive electric field pulses (application times T1b and T2b) as shown in FIG. 20C. As a result, the ink with the volume of three droplets can be allowed to fly from the nozzle hole 11. The times for application of the positive electric field pulse and the pulse interval times (rest times) of this case are represented as T1b=T2b=T3b=T4b=T5b. In other words, the times for application of the positive electric field pulse with a predetermined voltage for deforming and operating the actuator formed of each of the side walls 7 to allow the ink to fly from each of the nozzle holes 11, are set to be equal to each of the rest times between pulse application operations, during which the actuator is not driven. As a result, the ink can be discharged with efficiency.
FIG. 21 shows a relation between a fluctuation of a pressure P of each of the nozzle holes 11 and a drive voltage between the electrode 4 and the electrode 9. In FIG. 21, a time T1 corresponds to the time T1b of FIG. 19. FIGS. 22A-I to 22D-II each schematically show a behavior of each of the side walls 7, a change in pressure of each of the nozzle holes 11, and the ink flow path. FIGS. 22A-I to 22D-II are cross-sectional diagrams each showing the nozzle plate 3 and the head chip 26. FIGS. 22A-I, 22B-I, 22C-I, and 22D-I each show the nozzle plate 3 and the head chip 26 viewed from an axial direction of the nozzle holes 11, and FIGS. 22A-II, 22B-II, 22C-II, and 22D-II are side views thereof. FIGS. 22A-I and 22A-II each show a state obtained before application of the drive pulse in FIG. 21, FIGS. 22B-I and 22B-II each show a state at a time (time t11) when the drive pulse application is started in FIG. 21, and FIGS. 22C-I to 22D-II each show a state at a time (time t12) when the drive pulse application is finished in FIG. 21.
When the drive pulse is applied at the time t11, the pressure P of each of the nozzle holes 11 is rapidly changed into a negative pressure P1 simultaneously with the fluctuation (increase in volume) of each of the side walls 7 (see FIG. 22B-I and 22B-II). Then, the ink is gradually filled, and the pressure once returns to 0 (pressure P2). Further, the pressure fluctuates to a positive side by a force of a caused wave. When the ink is supplied from an ink supply port formed in the ink chamber plate 2, the pressure in the flow path is increased, and the pressure P becomes a peak value after the elapse of the time T1 (pressure P3) (see FIG. 22C-I and 22C-II) Then, when the drive voltage is returned to 0 at the time t12 after the elapse of the time T1 when the pressure P becomes the peak value, the side walls 7 are each returned to the original state shown in FIGS. 22A-I and 22A-II. When the volume of the ink is reduced as compared with that in the state shown in FIGS. 22B-I to 22C-II, the ink can be allowed to fly from each of the nozzle holes 11 with efficiency (FIGS. 22D-I and 22D-II). After that, the fluctuation in pressure of each of the nozzle holes 11 is repeatedly caused with a time twice as much as the time T1 being as one cycle, and gradually decreases.
However, in the case of the method of driving the ink jet head according to the related art by employment of drive waveforms shown in FIGS. 20A, 20B, and 20C, as apparent from the relation between the electric field applied voltage and the discharge speed shown in FIG. 23, there is a problem in that there is generated a discharge speed difference among one droplet, two droplets, and three droplets of ink. The discharge speed in each case of discharging the ink with the volumes of two droplets and three droplets is higher than that in the case of discharging one droplet. This is because an effect of the pressure change generated during the driving operation at the time T3b and the time T2b remains, and an remainder of a vibration due to the driving operation is added, thereby increasing the discharge speed. There is a problem in that, when the printing operation is performed by an ink-jet printer, the difference in discharge speed generated in this case is appeared as a difference in impact positions of ink droplets, thereby deteriorating an image quality of a printed material.